When a magnetic field is not present, a magnetic fluid, or ferrofluid, functions like a typical fluid, for example, taking the shape of a container in which it is stored. However when subjected to a magnetic field, the magnetic particles within the fluid align with the magnetic flux lines provided by an associated magnet. Magneto-fluidic seals, utilizing a magnetic fluid, are particularly useful for forming seals around shafts, for example rotating shafts such as a stirring shaft for a reactor or bioreactor, or a power delivery shaft. These magneto-fluidic seals are particularly useful for forming a hermetic environment for the exclusion of contaminants and preventing escape of biological matter from an enclosed space into the environment.
Conventional magneto-fluidic seals for shafts are formed between a pole piece and a sleeve affixed to the shaft. The pole piece includes an annular-shaped magnet defining north and south polarities of the pole piece. The pole piece and the sleeve are separated by a gap. Magnetic fluid fills the gap, forming a hermetic seal between the pole piece and the sleeve.
The magnetic fluid generally includes a suspension of dispersed magnetic particles coated with an anti-aggregation agent that forms a colloid. The magnetic fluid wears out when high magnetic fields are applied to the magnetic fluid over a long period of time due to clumping of the magnetic particles and loss of homogeneity, which decreases the reliability of the magneto-fluidic seal.
Conventional single stage magnetofluidic seals, for example, such as those illustrated in U.S. Pat. No. 5,954,342, include a non-magnetic housing, within which a magnetic system is installed, that includes a shaft and a ring shaped magnet. The magnet includes north and south poles, which abut the body of the magnet. The magnetic fluid is held in place by a magnetic field in a working gap. The magnet generates a magnetic field. The working gap includes magnetic field concentrators, such that the magnetic field intensity is the highest at locations where the magnetic fluid is positioned. The presence of the magnetic fluid, which, when magnetized, forms a structure somewhat analogous to an O-ring, provides for a sealing effect. Each such magnetic field concentrator is therefore a location of a ring-like arrangement of magnetic fluid, and each such ring-like arrangement therefore provides a sealing effect.
One problem with such conventional magnetofluidic seals is a relatively limited range of working temperatures, and a low reliability due to the presence of actual O-rings, which are typically made of rubber or a similar material. For example, if the working temperature range of the magnetic fluid is typically between 173 and 473° Kelvin (about −100° C. to about 200° C.), while the O-ring has a working temperature of 223 to 473° Kelvin (about −50° C. to about 200° C.), the effective working temperature range of the entire magnetofluidic seal structure is therefore 223 to 473 Kelvin.
Another conventional magnetofluidic seal includes a non-magnetic housing, ball bearings, a shaft, and a sleeve that includes several channels, into which permanent magnets are placed. Several such permanent magnets are typically located in the channel, such that, collectively, they form a ring-like structure, or a ring-like structure with gaps therein. Several such rows, or rings, of magnets can be used in a single magnetofluidic seal, with their constituent magnets typically of relatively simple shape, being evenly distributed throughout the circumference of the channel. The static gap between the sleeve and the housing is generally sealed using a conventional O-ring. However, this structure also suffers from the same disadvantage, and that the working temperature range of the entire structure is generally limited by the working temperature range of the O-ring.
Accordingly, there is a need in the art for a magnetofluidic seal with an expanded working temperature range, particularly one that is not limited by the working temperature range of the O-rings used in the structure.